Barbiturate dependent and nondependent rats will be used to explore the environmental and pharmacological determinants of the development and maintenance of barbital ingestive overindulgence. Excessive barbiturate ingestion will be induced as an adjunct to intermittent schedules of food delivery and the capacity of other adjunctive behaviors to substitute for barbiturate overindulgence will be evaluated in terms of the development, maintenance and alleviation of drug dependence.